Boiling water nuclear reactors contain discrete fuel bundles clustered together in the central portion of the reactor to form the steam generating core of the reactors. These fuel bundles have lower tie plates for supporting a group of upright fuel rods and admitting water moderator from the lower regions of the nuclear reactor. The bundles include an upper tie plate for maintaining the fuel rods upright and permitting water and generated steam to exit upwardly from the fuel bundle to the upper regions of the nuclear reactor. A channel surrounds both tie plates and the fuel rods extending therebetween to confine the flow path of the moderator between the tie plates and around the steam generating fuel rods. Additionally, fuel bundles contain fuel rod spacers distributed at vertical intervals from the bottom of the fuel bundle to the top of the fuel bundle. These spacers have the mechanical function maintaining the matrix of fuel rods in precise side-by-side relation. This prevents the otherwise flexible fuel rods from coming into abrading contact under the forces of the upward hydraulic flow as well as maintaining the fuel rods in their designed side-by-side relation for improved nuclear performance.
Operation of the fuel bundles within the reactor can be described in terms of both thermal hydraulic performance and nuclear performance.
In terms of thermal hydraulic performance, moderator in the liquid state enters the bottom of each fuel bundle through the lower tie plate, and flows upwardly within the channel and between the fuel rods. During this upward flow increasing amounts of vapor (steam) are generated. At first and in the lower portion of the fuel bundle, liquid flow predominates with an upwardly increasing array of vapor bubbles. Later and in the upper extremities of the bundle, vapor flow predominates with liquid forming an increasingly reduced fraction of the upward moderator flow.
In order to maintain stable boiling within the fuel bundle, it is necessary that each of the fuel rods be coated with a film of liquid moderator (water) during the operation of the reactor. This film of water is particularly critical in the upper two phase (steam and water) region of a boiling water nuclear reactor. Further, the vapor fraction in the upwardly flowing moderator tends to increase to and towards the center of a fuel bundle. Therefore, it is the fuel rods in the central upper portion of the fuel bundle that are particularly critical when it comes to maintaining a film of water present over the surface of the fuel rods.
It has been found that where this film is not present over the surface of a fuel rod, a phenomenon known as boiling transition can occur. Simply stated, in areas of boiling transition, the wall temperature of the fuel rods rapidly rises. Both the long term metallurgical life of the fuel rod cladding as well as the short term mechanical containment of the fuel rods is threatened by boiling transition. For this reason, it is well accepted in the nuclear industry that the absence of film coating the fuel rods in any portion of the fuel bundle of a boiling water nuclear is to be avoided during reactor operation.
The fuel rod spacers distributed at the vertical intervals interior of the fuel bundles have a thermal hydraulic function. It has been found that these spacers cause augmentation of the necessary water film on the fuel rods immediately downstream of the moderator flow within the fuel bundle. In order to understand this phenomenon, some definition of terms and remarks about the complex and little understood phenomenon of boiling within a boiling water nuclear reactor should be made.
First, and regarding the term "downstream of the spacer", the reader will understand that moderator flows from the bottom of the fuel bundle to the top of the fuel bundle. Therefore, the region that is downstream from the spacer is that volume of the fuel bundle immediately above the spacer. Further, it will be understood that it is likely that the region immediately upstream of a spacer (or immediately below the spacer) near the top of a fuel bundle is most likely to have a lack of liquid film coating the fuel rods. These regions of the fuel bundle will most likely be subject to boiling transition.
Second, the problem of boiling relates at least to the interaction of four highly complex variables. These variables are film flow over the surface of the fuel rods, vaporization from the film on the fuel rods, entrainment of liquid film on the fuel rods within the upwardly flowing vapor, and finally deposition of liquid droplets from the upwardly flowing liquid droplets upon the fuel rods to help maintain the film. Simply stated, it has not been possible to accurately predict the interaction of these variables; design of boiling water nuclear reactor fuel bundles and spacers requires considerable testing of actual models in the form of full scale test assemblies.
Finally, and because of the unpredictable interaction resulting in the boiling phenomena of boiling water nuclear reactor fuel bundles, it has been found that different designs of the spacers placed in the upper two phase region of the fuel bundle restore the necessary film coating to different degrees. Thus it will be understood in the following discussion, that the type and total number of spacers in a particular fuel bundle can vary.
Regarding reactor nuclear performance, in a boiling water nuclear reactor, the density of the water is important. Simply stated, the nuclear reaction generates fast neutrons. The continuance of the nuclear reaction requires slow or thermalized neutrons. It is the function of the moderator to moderate the fast neutrons to the thermal state so that the reaction can continue.
The sufficiency of this moderation is a function of the density of the moderator at any particular point within the interior of the reactor. As has already been mentioned, moderator density in the central upper region of the fuel bundle is low.
To correct this condition, it is well known to insert so-called water rods in the interior of a boiling water nuclear reactor fuel bundle. These water rods are filled with liquid moderator to supply to the upper region of the fuel bundle the necessary moderator density for the efficient nuclear reaction.
It is to be understood that conventional water rods, while having the nuclear efficiency of supplying water moderator to the upper central portion of the fuel bundle, have a thermal hydraulic deficiency. Specifically, and in order that the water rods remain full with liquid moderator, water is taken from the bottom of the fuel bundle and shunted directly through a heater rod to the top of the fuel bundle. The water bypasses the steam generating flow within the fuel bundle and to that extent is inefficient in its upward flow through the fuel bundle.
It is the purpose of this invention to provide a fuel bundle with an improved water rod that both continues the water density in the upper two phase region of fuel bundles having water rods and yet improves the thermal hydraulic characteristics of the fuel bundle.